Re: [PATCH 4/5] rust: hrtimer: use READ_ONCE instead of read_volatile

[Andreas Hindborg <a.hindborg@kernel.org>]

Boqun Feng <boqun.feng@gmail.com> writes:

> On Wed, Jan 07, 2026 at 11:11:43AM +0100, Andreas Hindborg wrote:
>> FUJITA Tomonori <fujita.tomonori@gmail.com> writes:
>> 
> [...]
>> >>>
>> >> 
>> >> This is a potentially racy read. As far as I recall, we determined that
>> >> using read_once is the proper way to handle the situation.
>> >> 
>> >> I do not think it makes a difference that the read is done by C code.
>> >
>> > What does "racy read" mean here?
>> >
>> > The C side doesn't use WRITE_ONCE() or READ_ONCE for node.expires. How
>> > would using READ_ONCE() on the Rust side make a difference?
>> 
>> Data races like this are UB in Rust. As far as I understand, using this
>> READ_ONCE implementation or a relaxed atomic read would make the read
>> well defined. I am not aware if this is only the case if all writes to
>> the location from C also use atomic operations or WRITE_ONCE. @Boqun?
>> 
>
> I took a look into this, the current C code is probably fine (i.e.
> without READ_ONCE() or WRITE_ONCE()) because the accesses are
>
> 1) protected by timer base locking or
> 2) in a timer callback which provides exclusive accesses to .expires as
>    well. Note that hrtimer_cancel() doesn't need to access .expires, so
>    a timer callback racing with a hrtimer_cancel() is fine.
>
> (I may miss one or two cases, but most of the cases are fine)
>
> The problem in Rust code is that HrTimer::expires() is a pub function,
> so in 2) a HrTimer::expires() can race with hrtimer_forward(), which
> causes data races.
>
> We either change hrtimer C code to support such a usage (against data
> races) or change the usage of this HrTimer::expires() function. Using
> READ_ONCE() here won't work. (Yes, we could say assuming all plain
> writes on .expires in C are atomic as some other code does, but hrtimer
> doesn't rely on this, so I don't think we should either)

I don't think we should change the C code, I think the Rust API is
simply wrong. The function should have same constraints as
`forward_now`, ie. call while having exclusive access to the timer
(during setup for instance), or in callback context.

We should change it to take `self: Pin<&mut Self>` and add it on
`HrTimerCallbackContext` as well.

@Tomo, do you know of any users of this function?


Best regards,
Andreas Hindborg